Mykiss walks down the hall, stops and sniffs, then turns the corner into the master bedroom. He climbs onto a pile of pillows lying on the floor, then pauses, crouches, and makes a flying leap onto the bed. After finding his owner's chin and giving it a friendly nip and kiss, he jumps onto the nightstand and samples the contents in the glass standing there. Having exhausted all interesting possibilities on top of the bed, Mykiss climbs down and runs to his favorite spot beneath to rest.
Oh, another full morning in the life of a pet rat! Our friends are always eager to explore and experience their surroundings. How do such small animals orient themselves in such a large world? We humans rely primarily on our vision to get around. As mammals, rats and humans possess identical structures in their eyes, but the similarity ends there. Although rats utilize their visual sense, the cues it provides them are much different than what we experience.
The young rat's eyes begin to open 14-17 days after birth, and may actually be used to determine its age in the absence of other information. The rat eye, like that of all mammals, is composed of four primary parts: the cornea, iris, lens and retina. The cornea is the outermost transparent layer of the eye, through which light enters. Behind the cornea lies the iris, which is a thin, circular-shaped screen perforated by the pupil. The pupil serves as an aperture and functions to regulate the amount of entering light by either increasing or decreasing in diameter. Pigmentation of the iris is what gives the eye its color. Albino rats have pink eyes not because there is pink pigment in the iris; rather, it is the absence of color in the iris that allows the reddish retinal blood vessels in the back of the eye to be visible. The transparent biconvex body of the lens lies behind the pupil, and it serves to focus light on the retina. The retina occupies the rear-most position in the eye, and it is the part that contains photosensitive components of the eye -- the rods and cones. The retina is nervous tissue (actually an extension of the brain!), which receives signals generated by external objects and transmits them, via the optic nerve, to the brain for interpretation. On the exterior of the eyeball lie six muscles that function to move it in all directions.
Do rats perceive color images like we do? Interestingly, color vision in mammals is rare, the exception being tree-dwelling and fruit-eating species like squirrels and primates. It is believed that the ability to perceive color aids these animals in obtaining food. In the retina, there are two types of photoreceptors: rods and cones. Rods are detectors of tone, i.e., gradations of lightness and darkness, and cones are sensitive to color. In rats, rods are the primary photoreceptor, while cones are scarce (less than 1% of total photoreceptors). Although the cones in the rat eye are shown to have sensitivity to green and UV light, behavior experiments have demonstrated that rats lack color vision.
Rats have laterally-placed eyes that sit on the sides of their heads. This placement allows very little overlap of the visual field of each eye. Panoramic rather than binocular vision is the rule -- panoramic vision gives the rat a wide uninterrupted field of view; this likely helps it to escape predators, among other things. Rats do have some binocular ability that is limited to distant objects. Binocular vision is the fusion of separate images captured at slightly different angles by each eye, which makes depth perception possible. In addition, rat eyes are unable to accommodate (successfully image objects at different distances), and it is believed that most small animals have such poor vision that these fine adjustments are not really worthwhile, anyhow.
Part of the reason why rats have such poor eyesight is because generally the eyes of small and nocturnal animals are far-sighted, which signifies a lack of need for optical quality. Like most nocturnal animals, rats depend less on vision than do diurnal (active in the daytime) ones. Rather, rats have sacrificed much visual capacity and instead rely more on olfactory (smell), auditory (hearing), and tactile (touch) abilities. This phenomenon is clearly illustrated in the observance of certain strains of rats that undergo spontaneous retinal degeneration soon after birth. Compared to normal, sighted rats, these animals show no differences in behavior under daylight conditions. Compare this behavior to that of a blind bird or kitten, both of which would be virtually helpless without sight.
Although visual acuity is poor, the rat has a large cornea and pupil, as well as a powerful lens. Coupled with the rod-dominated retina, these components result in a very light-sensitive organ that is a good detector of movement. It has been demonstrated that neither the sight nor smell of a cat will produce defensive reactions in a rat, but movement of a cat or an inanimate object (a card, for example), will result in defensive posture. This suggests that motion of the predator, rather than its visual image, is a major factor in the initiation of defense responses in the rat. In certain instances, however, rats do use vision for short-range purposes. For example, wild rats have been observed to discriminate between sizes of sunflower seeds, choosing long over short seeds. Young rats at play with one another use visual rather than tactile (from whiskers) cues to orient themselves before attacking one another's back. In the instance that one rat steals a food pellet from the other, the victim also responds from visual cues.
When using their vision for orientation, rats are guided by more distant objects rather than nearby ones. The biologist M.T. Block stated that nearby objects are not the most significant for rat vision because At close range the rat has his nose and [whiskers] to fall back on and only uses vision as a last resort. He went on to state that the rat's visual system functions to orient it in space, to recognize relative distance and direction, and that recognition of objects and forms is possible but secondary to the coordinate system. To support Block's hypothesis, it has been shown that rats trained to run in an open-air maze are guided by cues outside the maze, rather than inside. In experiments where the original maze was rotated 90-180 in a room, rats performed poorly. Apparently, they were guided by spatial cues, such as light from a window or patterns on the ceiling or wall. Once these cues were moved, the rats were literally lost!
Back to Mykiss' trip through the apartment: Perhaps the sun streaming from the bedroom window is what guided him down the hallway, and he climbed onto the pillow pile using primarily tactile cues. Due to lack of depth perception at close range, he overcompensated and made a very long jump onto the bed, then used his sense of smell to find his loyal owner. After quenching his thirst, he again used spatial cues to safely jump from the bed, then his olfactory sense guided him to his well-marked sleeping spot. Our rat friends utilize sensory information that we are not capable of detecting nor discerning, and this helps guide them through their day. No, they don't see what we do, but they are aware of their surroundings in a way that suits their lifestyle just fine!